MARYLAND WEATHEE SEEVICE 31 



the latter is modified in its influence by local differences of soil, 

 topography, etc. The rainfall, for example, is practically uniform 

 over the whole of Worcester County, yet the rain which falls upon 

 the dunes at Ocean City soon sinks to a level at which it is unobtain- 

 able by the vegetation, while that which falls upon the clay soils in 

 the interior of the county is retained in the surface layers. Im- 

 portant to vegetation, but in a less degree, are humidity, sunshine, 

 wind, fog and other elements of the climate. 



Temperature. — The most fundamental influence of temperature in 

 an area is the excluding of all plants except those whose ecological 

 optimum of temperature approaches the mean temperature of the 

 growing season of the area. The native, introduced, and cultivated 

 plants of Maryland find in its mean temperatures such as are more 

 or less closely approximate to their optimum ; plants of the tropical 

 and sub-tropical regions to the south are excluded from occurrence 

 either by the direct fatal effect of freezing temperatures or by reason 

 of the relative shortness of the growing season to their habits of 

 growth, flowering and setting seed. Plants of northern and arctic 

 regions are excluded either by reason of the competition of the more 

 accurately adapted plants, or, as has been suggested by Aierriam*, 

 by the harmful effect of the temperatures of a few weeks in the hot- 

 test part of the summer. A most important end-effect of the 

 temjierature is, then, the determination of the character of the 

 flora,- — there are certain strictly tropical families and genera, and 

 others which occur solely or chiefly in the temperate or the arctic 

 regions, a matter, however, into which the geological history of floras 

 niters as an important consideration. 



A second influence of temperature is the regulation of the periodic 

 phenomena of plants, — the date of germination for annuals or of 

 leafing-out for perennials, the date of flowering, of the ripening of 

 fruit or seed, the date of leaf -fall or death, etc. The annual curve of 

 temperature for all regions lying within the North Temperate Zone 

 rises during the spring and summer months to a maximum and falls 

 in the autumn and winter to temperatures which may be below those 



*Merriam, C. Hart, Laws of the Temperature Control of the Geographical 

 Distribution of Terrestrial Animals and Plants, National Geographical Mag- 

 azine, Vol. VI., 1894, pp. 229-23S. 



